Process of bleaching a sheet of groundwood pulp



nited States Patent C) 3,423,283 PROCE @F BLEACHTNG A SHEET F GROUNDWQOD BUILT Herman R. Goodwaid and Dale L. Schechter, Midland,

Mich, assignors to The Dow Chemical Company, Midland, Mich, a corporation of Deiaware No Drawing. Filed Sept. 2, 1965, Ser. No. 484,727

U.S. Cl. l6271 6 Claims lint. Cl. 1021c 9/16; DZllf 11/00 ABSTRACT OF THE DTSCLUSURE A process for bleaching groundwood pulp which comprises: contacting a groundwood pulp mass in the form of a damp sheet with an aqueous bleaching solution, subjecting the so-treated sheet to a temperature of from about 130 to 230 C., and drying said sheet.

This invention relates to the bleaching of wood pulp and more particularly relates to a novel method of rapidly bleaching groundwood or mechanical pulp to obtain high increases in brightness.

Because of the high yields obtained from the grinding operation compared with other pulping processes and the relatively low production costs, groundwood, or mechanical pulp as it is called, is the preferred raw material for the production of lower grade printing paper, such as, for example, newsprint. Moreover, with the introduction in the past few years of several modern bleaching processes, a significant broadening of the range of papers in which groundwood could be used has been made possible. These bleaching processes, which include the use of, for example, sodium or zinc hydrosulfite, hypochlorites, and various peroxides as bleachants, are normally carried out in vats or in so-called continuous bleaching towers. In the towers, especially in peroxide bleaching, the pulp is first dewatered or thickened to a density or consistency ranging from, for example, about 1.5 to 50 percent and higher in solids, whereupon the pulp is pumped to the towers. Here it is contacted with the bleachant at, for example, temperatures of from about 60 to 80 C. and at a certain pH for a particular retention time, suflicient to permit completion of bleaching. Normal times for bleaching by these processes to increase brightness using, for example, peroxides, are within the range of from several minutes to several hours, depending on the method employed and conditions. The bleached pulp is then discharged from the towers, neutralized by washing, and dried. An increased brightness over those exhibited by untreated pulp may be obtained by bleaching in the above manner.

Since brightness is usually referred to in the pulp industry as G.E. points of brightness (spectrophotometers by the General Electric Company being commonly used in the determination) the latter designation will be used hereinafter.

Notwithstanding these past advances in bleaching and brightening groundwood, disadvantages still exist in that extended bleaching times are normally required. Accordingly, there is a need in the art for a high speed method of significantly increasing the brightness of mechanical pulps.

A principal object of the present invention is to provide a novel method for bleaching groundwood pulps in a rapid manner.

Another object is to provide a novel, high speed onmachine, method for bleaching mechanical pulp wherein unusual increases in brightness are obtained.

Other objects and advantages will become more apparent from the description of the invention which follows.

ICC

The novel method of the present invention comprises in general the steps of: first providing a groundwood pulp as a slurry thereof, said slurry having a consistency or density of solids (termed herein as density) of, for example, from about 1.5 to about 50 percent or more by weight of the dry pulp. The slurry is preferably formed into a continuous sheet by means, for example, of conventional paper making machinery, and dewatered, preferably to leave it damp; whereupon, the damp sheet is contacted or treated with a bleaching solution by, for example, spraying means or a dip method. So-treated, the sheet is then exposed to a temperature within the range of from about to about 230 C. By so-doing, the sheet is rapidly bleached to unusual magnitudes of brightness in a matter of seconds, whereupon, it is dried by continued exposure at said temperature. If desired, the bleaching solution may be applied as the sheet is exposed to said temperature.

The actual contact time for the bleachant with the pulp is, ordinarily, from about 0.5 to about 1.5 minutes, and preferably from about 0.75 minutes to about 1.25 minutes at a temperature of from about C. to about C. Drying of the sheet exposed to the above temperature range, partially during and immediately after bleaching, is accomplished within about 2 minutes, and generally within about 1% minutes.

Groundwood pulps which may be treated by the novel process of the present invention include, for example, those of spruce, poplar, eastern balsam and the other common g-roundwoods. Spruce and poplar groundwoods produce excellent results and are preferred.

The slurry from which the sheet is to be made will preferably have an intermediate (for example, 10-15 percent) to high density (for example, 20 percent or above). However, low density slurries (for example, 36 percent) may also be employed. The resulting damp sheet to be treated will preferably have a density or solids content of, for example, about 50 to about 70 percent.

The bleaching solution which may be used includes the various conventional peroxide bleaches, such as, for example, sodium peroxide or hydrogen peroxide. These bleaching solutions should have an adjusted pH of about 10.5 to 11.5 since below and above this range the brightness increases obtainable diminish significantly. Preferably, the peroxide bleachant will contain from about 1 to about 5 percent peroxides as an aqueous solution thereof having an adjusted pH within the range of from 10.8 to 11.2. Other conventional bleaches such as, e.g., hydrochlorite, hydrosulfite, or bisulfite agents, when employed will be used at concentrations which may be readily determined by one skilled in the art. Accordingly, the term bleaching solution as used herein includes aqueous solutions containing one or more of these bleaching materials.

Bleaching and heating of the pulp, preferably as a continuous sheet, is done not only to accelerate the bleaching action but also is done to permit rapid drying. By so doing the method of the present invention readily lends itself to on-machine operations. Accordingly, a good heat transfer mechanism is essential such that the heat be supplied to the sheet as rapidly and uniformly as possible. This may be done, for example, by providing a heated atmosphere having a large temperature differential (AF) with the damp bleached for bleaching sheet, or by means of steam heated rollers. Nothwithstanding, the elevated temperature to which the sheet is exposed, it is probably in effect thermostated to about 100 C. due to the water present therein, until said water is all dissipated.

The novel method of the present invention in addition to providing a very high speed bleaching of groundwood pulp also provides unusually large brightness increases of a magnitude heretofore unknown. For example, brightness increases of from approximately 12 to about G.E. points over the brightness shown by the untreated pulp are obtained, depending on the groundwood employed and the process conditions. Results and advantages, such as obtained herein, are surprising and unexpected in that it has not heretofore been apparent or suggested that such large increases in brightness could be obtained from bleaching in such a high speed rapid manner.

The following examples further illustrates the novel method of the present invention and are not to be construed as limiting the invention thereto.

EXAMPLE I A slurry of poplar groundwood was formed into a paper sheet, blotted to remove most of the moisture, thereby having it damp without containing excess water, then further dampened with enough of a conventional peroxide bleach solution at pH 11 to constitute a 2 percent sodium peroxide bleach (2 grams of Na O per 100 grams of oven dried pulp) at a consistency of about percent for a contact time of the bleach with the paper of about 2 minutes. (Consistency is defined hereinabove and after as the percentage of oven dry pulp present in the wet mixture.) This damp sheet was quickly placed in a 170 C. oven for 2 minutes, whereupon, the sheet was substantially dried. Brightness tests were then conducted using a spectrophotometer. The brightness of the starting pulp, when formed into a 3 /2 inch diameter paper and air dried, registered about 54 G.E. points, while that of the treated sheet registered about 76 G.E. points. The significant and unusually large difference, therefore, attributable to the present novel bleaching process was an increase of about 22 G.E. points, the bleaching itself not having taken over about 2 minutes.

EXAMPLE II By repeating the method used in Example I above, eastern balsam groundwood pulp exhibiting about 55 G.E. points of brightness was treated with a 2 percent sodium peroxide solution at 200 C. for 150 seconds, and again tested for brightness upon being dried. The resulting product had a brightness of about 68 G.E. points, i.e., and increase of 13 points.

EXAMPLE III The procedure and conditions of Example I were duplicated except that in this case spruce groundwood was treated. A brightness increase of the treated sheet over the starting pulp of about 12 G.E. points was obtained.

EXAMPLE IV With respect to bleaching groundwood pulps with sodium peroxide as a bleachant by the present novel process and to illustrate the effect of the peroxide concentration in bleaching, a number of poplar groundwood sample papers (damp) were prepared as follows: A slurry of poplar groundwood containing 0.73 g. of pulp (oven dry basis) was formed into a paper sheet on a Buchner funnel, then either placed on the oven heat block for a predetermined time and temperature or blotted to the desired consistency. Each sheet was then tested for brightness. The sheets were dampened with 2 cc. of the peroxide bleach solution containing enough Solozone bleach solution prepared as outlined in Du Ponts bulletin Peroxide Bleaching of Groundwood 1952) and adjusted to the desired pH with H 80 to constitute the desired peroxide concentration. The bleach solution was first roughly distributed over the surface of the paper sheet and then dispersed evenly by quickly running a roller back and forth over the sheet. The wet sheet was then either placed on a massive block of nickel metal in an oven or suspended in the oven at a predetermined temperature and time. After the bleaching period in the oven was terminated, the sheets were either rinsed or not and then allowed to dry (or continue to dry) at room temperature. In some cases, after water rinsing, the sheets were placed back in the oven to speed up the drying process. After drying, the brightnesses were determined and compared with the brightness data obtained aforesaid of the untreated pulp. The conditions of these tests and results are given in Table I below.

TABLE I NazOa Bleach Bright- Increase ness (G.E. in G.E. Ex. Percent pH Temp. Time units) points of C.) (min.) (after brightness bleach) 2. 5 13. 0 170 2. 0 70. 10 15.50 2. 5 12. 5 170 2. 0 74. 55 19. 95 2. 5 12. 0 170 2. 0 77. 23. 20 2. 5 11. 5 170 2. 0 78.05 23. 2. 5 11. 0 170 2. 0 79. 15 24. 2. 5 10. 5 170 2. 0 76. 25 22.05 2. 5 l0. 0 170 2. 0 73. 25 19. 05 2. 5 9. 5 170 2. 0 70.00 15. 80 2. 5 9.0 170 2. O 65.85 11. 65 5. 0 13. 0 170 2. 0 77. 10 22. 90 5. 0 12.0 170 2. 0 79. 15 24. 05 5.0 11. 0 170 2. 0 81. 55 27. 35 5. 0 11. 0 170 2. 0 81. 50 27. 30 5. 0 11. 0 170 2. 0 80. 00 25. 80 3. 0 11. 0 170 2. 0 79. 50 25. 30 2. 5 11. O 170 2. 0 78. 25 24. 05 2. 0 11. 0 170 2. 0 75. 65 21. 45 1. 5 11. 0 170 2. 0 72. 25 18.05 1. 0 11.0 170 2.0 65.90 11.70 0.5 11. 0 170 2. 0 62.35 8. 15

The present invention can be modified and changed without departing from the spirit or scope thereof and it is understood that the invention is only limited as defined in the appended claims.

We claim:

1. In a method of bleaching groundwood pulp to a high brightness comprising contacting a sheet of said grOundwood with a bleaching solution, at an elevated temperature, the improvement which comprises: heating the sheet so-contacted to a temperature of from about C. to about 230 C. for a period of from about 0.5 to about 1.5 minutes to rapidly bleach said sheet, and drying said sheet so-bleached.

2. The method of claim 1 wherein the sheet is contacted with said bleaching solution for a period of from about 0.75 to about 1.25 minutes at a temperature of from about to about C.

3. The method of claim 1 wherein the groundwood pulp is spruce or poplar.

4. The method of claim 1 wherein the bleaching solution is an aqueous peroxide bleaching agent of from about 1 to about 5 percent in concentration.

5. The method of claim 1 wherein the pH of the bleaching solution is within the range of from about 10 to about 13.

6. The method of claim 1 wherein a damp sheet of said pulp is contacted with said bleaching solution.

References Cited UNITED STATES PATENTS 2,125,634 8/1938 Heritage 16213 2,150,926 3/1939 Kauffmann et al 162-13 2,510,595 6/1950 McEwen et al. 162-13 OTHER REFERENCES Blackerby, Whats New In Bleaching, Pulp and Paper, vol. 38, No. 30, September 1964, pp. 38-41.

S. LEON BASHORE, Primary Examiner.

US. Cl. X.R. 

